Delivery of a full course of vaccine in a single dose has held attraction in both human and veterinary medicine and a number of patents and other publications (e.g. U.K. Patent No. 1,567,503) have addressed this possibility. For veterinary applications, the advantages include:
(i) reduced time--animals need be handled only once, PA1 (ii) reduced cost--single veterinary visit and reduced handling costs, PA1 (iii) guaranteed compliance with recommended dose schedule (number of doses, time interval between doses). PA1 (i) exposure of biological materials to denaturing chemical and physical conditions, and PA1 (ii) difficulty of scale-up, PA1 (iii) low efficiency of incorporation of hydrophilic compounds (e.g. proteins). PA1 (i) the immunogen is held in a selected configuration during the drying process, PA1 (ii) adjuvant is available to stimulate the immune system at every pulsed release, PA1 (iii) during in vivo residence time, whilst delayed-release polymer is undergoing biodegradation, the immunogen is protected from thermal and perhaps enzymic denaturation by attachment to a solid support.
In human medicine, the above three advantages are also important with compliance being extremely important in developing countries where repeated access to infants is often not possible. In addition, the pain and suffering associated with vaccination, especially of infants, is an additional reason to favour a single-dose vaccine in human medicine.
Early studies of vaccination using inactivated vaccines (generally tetanus or diphtheria toxoids), have demonstrated the importance of two or more discrete doses of vaccine with an interval of at least 4 weeks, and preferably longer, between doses. A third dose is sometimes necessary to induce an adequate immune response, especially in young animals or infants where transfer of maternal antibodies could interfere with the preliminary immune response.
Recent studies in theoretical immunology have supported these findings and introduced the phrase "affinity maturation". Affinity maturation describes the process whereby plasma cells secreting high affinity antibodies to the desired immunogen are preferentially selected whilst plasma cells secreting antibody of lower affinity are lost. The process involves competition between follicular dendritic cells and plasma cells for antigen binding and thus can only occur effectively in the presence of limiting amounts of antigen. The process of affinity maturation may not commence until 2 to 3 weeks after a primary vaccine dose and it is important that the second dose of antigen not be given until the process is effectively complete. This is readily achievable in a multidose vaccination schedule provided the first dose does not contain too much antigen. However, for this process to be achieved in a single dose delayed-release vaccine, it is important that the second and subsequent doses do not release their antigen payload prematurely. To achieve this, the antigen must be contained within a matrix which has a defined time of degradation. This matrix should be biodegradable, although biocompatible matrixes have been proposed as acceptable. A number of options have been reviewed by Cox & Coulter, 1992.
The major effort to develop delayed release vaccines has centred round the studies of Eldridge et al., 1990; 1991, who used the biodegradable copolymer--polylactide coglycolide to produce antigen-containing microspheres and observed a delayed-release of the antigen contents in vivo (see also Australian Patent Specifications Nos. 79929/87 and 33433/89). Similar observations have been reported by Kreuter, 1990 using nanoparticles produced from acrylate polymers. Although the above workers were able to show that the concept of delayed-release vaccines was possible, the process they used in the preparation of the vaccines suffered from a number of deficiencies making it unsuitable for the routine manufacture of a vaccine. The major problems were:
In European patent publication No. 0486959, in the name of Vectorpharma International SpA, there are disclosed controlled release, particulate pharmaceutical compositions containing pharmacologically active substances, the compositions comprising a biodegradable polymer such as polylactic acid, polyglycolic acid and copolymers thereof and/or other polymers including a polysaccharide gellifying and/or bioadhesive polymer, an amphiphilic polymer, an agent modifying the interface properties of the particles and the pharmacologically active substance. In the preparation of the pharmaceutical composition, the polymeric substituents are co-solubilised with the agent modifying the interface properties either in the absence of any solvent or in the minimum necessary amount of solvent, and the pharmacologically active substances then dissolved or dispersed in the polymer solution prior to formation of the final particles, for example by emulsion, extrusion, spray drying or spray-congealing techniques. As previously described, this technique suffers from a major disadvantage in that the pharmacologically active substance is directly exposed to the mixture of polymeric compounds together with any solvents therein, which results in the denaturing of biological materials used as the pharmacologically active substance.
It is a principal object of the present invention to provide a vaccine preparation and method for the production thereof wherein the immunogenic material is not exposed to an organic solvent or other organic phase when in soluble form, so as to ensure that there are no conformational changes in the immunogen, in other words to maintain the native structure of the immunogen.